Proximity sensor devices (also commonly called touch sensor devices) are widely used in a variety of electronic systems. A proximity sensor device typically includes a sensing region, often demarked by a surface, in which input objects can be detected. Example input objects include fingers, styli, and the like. The proximity sensor device can utilize one or more sensors based on capacitive, resistive, inductive, optical, acoustic and/or other technology. Further, the proximity sensor device may determine the presence, location and/or motion of a single input object in the sensing region, or of multiple input objects simultaneously in the sensor region.
The proximity sensor device can be used to enable control of an associated electronic system. For example, proximity sensor devices are often used as input devices for larger computing systems, including: notebook computers and desktop computers. Proximity sensor devices are also often used in smaller systems, including: handheld systems such as personal digital assistants (PDAs), remote controls, and communication systems such as wireless telephones and text messaging systems. Increasingly, proximity sensor devices are used in media systems, such as CD, DVD, MP3, video or other media recorders or players. The proximity sensor device can be integral or peripheral to the computing system with which it interacts.
One issue with some past proximity sensor devices is effects of interference on the sensor device. In particular, difficulties have long been realized in identifying and reducing the effects of noise and other interferers upon the sensor device. Interference can originate from various sources, including display backlights, power supplies, wireless communication devices and the like. Although many sensors now include filtering that can effectively remove many types of interference, problems remain in identifying and removing interference with frequencies that are close to the sensor sensing frequency or any of its harmonics. These types of interferers are sometimes referred to as “tuned noise”, and are difficult to identify or filter out. Specifically, because the effective “beat” frequency of the tuned noise is often very close to the frequency of signals resulting from user-applied stimulus the tuned noise will often appear as a stimulus applied to the sensing region. As a result, distinguishing the effects of undesirable tuned noise from the desirable effects of the stimulus can be quite difficult. Further, the time to observe tuned noise can be significant because the beat frequencies of such noise can be relatively low (e.g. on the order of 10 Hz or less), and therefore the period of an entire beat cycle can be significant (e.g. on the order of ten seconds for a beat frequency of 0.1 Hz). Further, in some instances the beat signal may be indistinguishable from a user input.
Nevertheless, several techniques for reducing the effects of interference, including tuned noise, have been attempted. One conventional interference avoidance technique involves comparing the output signals produced by operating the sensor at two or more different sensing frequencies when no stimulus is present on the sensing region, and then subsequently operating the sensor using the frequency that produces the lesser amount of interference. This technique has several disadvantages, however, in that determining whether the stimulus is present at any given time can be difficult in practice, particularly in the presence of significant external interference. Moreover, because this technique measures interference only when the stimulus is not present, sources of interference present within the stimulus itself (e.g. environmental radio frequency noise coupled to the sensor via the stimulus) are not considered.
Accordingly, it is desirable to provide systems and methods for quickly, effectively and efficiently detecting interference in a proximity sensor device, even when the frequency of the interference noise is close to the sensing frequency. Other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.